1. Field of the Invention
The present invention is related to a combined communications system having multiple communications systems including a new communications system and an old communications system, of which system bandwidths overlap at least partially.
2. Description of the Related Art
Wireless communications are utilized in various applications such as mobile telephone, wireless LAN, broadcasting (terrestrial, satellite), radar, and positioning. It is anticipated that frequency allocations will be considered with respect to these existing systems and also in the future with respect to new systems such as the third generation (3G) mobile communications system-evolved long-term evolution (LTE) system, which is being considered in 3GPP, and the fourth generation mobile communications systems. In addition, with a trend toward higher-speed mobile communications systems in recent years, there is also a trend toward an increased bandwidth required for allocation.
On the other hand, there is a limit to wireless frequency resources. Moreover, frequency utilization varies among countries and regions. Taking into account such circumstances as described above, it will not be easy to uniformly allocate to all countries and regions, exclusively for new systems in the future, a wide, single bandwidth. Moreover, the propagation characteristics vary from one frequency band to another. Thus, it is anticipated that, depending on applications, the degree of freedom in allocating a required frequency band will be somewhat limited, making frequency allocations more difficult.
Then, a scheme to ensure that existing systems are not affected even in currently-utilized frequency bands may be introduced to allow allocating frequencies to new systems, and utilizing frequencies more efficiently.
A related-art method as disclosed in Non-patent document 1 (Ultra-wideband radio technology: potential and challenges ahead. Porcino, D., Hirt, W.; Communications Magazine, IEEE Volume 41, Issue 7, Jul. 2003 pp. 66-74), as a scheme of sharing frequencies between an old system and a new system, spatially limits the use by the new system to indoors, and fixes the limits of the transmit power level according to the existing system.
In a related-art method as disclosed in Non-patent document 2 (IEEE 802.11 WG, Part 11: Wireless LAN Media Access Control (MAC) and Physical layer (PHY) specifications, Amendment 5: Spectrum and Transmit Power Management Extensions in 5 GHz band in Europe, October 2003), as in DFS, which is an IEEE 802.11h standard, signals are detected over a predetermined time period prior to transmission of traffic data, whether the existing system uses the band in question is checked, and the traffic data are transmitted using a band not used by the existing system.
As disclosed in Non-patent document 1, in a scheme such that the location of use of the frequency band as well as the transmit power level are restricted in a fixed manner, even when there are no existing terminals in the vicinity that may be interfered with, the location of use and the transmit power level is restricted, which is not desirable from the point of view of effectively utilizing frequencies. Frequency resources available for a new system could be severely limited especially when the utilization of frequencies vary from one region to another or existing communications systems which are not frequently used use various frequency bands.
Moreover, as disclosed in Non-patent document 2, with a scheme of detecting whether an existing system is being utilized in the vicinity to avoid interference, when the existing system is a broadcast receiver or a satellite-system receiver, effectively avoiding interference may be difficult. This means that resources allocatable to a new system may be very limited. This is because, for example, regardless of whether a receiver of the existing system operates in the vicinity of each transmitter, the new system operation is restricted to the whole of the large broadcasting area. This aspect is also not desirable regarding effectively utilizing frequencies. Moreover, for seeking to detect the receiver operation of the existing system, as the receiver itself of the broadcasting system does not transmit any signal, it is simply difficult to appropriately detect the receiver of the broadcasting system with the new system.